Passive infrared intrusion detectors are being used for monitoring spaces, e.g. in museums, banks and industrial areas. Such detectors detect infrared body radiation from intruders, in a wavelength range from approximately 6 to 15 .mu.m. They consist essentially of a housing with an entrance window which is transparent to the infrared radiation, focusing optics, one or more infrared sensors, and an electrical signal evaluation and alarm activating circuit. The entrance window typically consists of infrared-transparent polypropylene or polyethylene. If an intruder enters the area monitored by the detector, his infrared body radiation enters through the entrance window into the intrusion detector and is focused by the focusing optics onto the infrared sensors. The infrared sensors output a signal to the evaluation circuit which amplifies the signal and compares it to a predetermined threshold. If the threshold is exceeded, an intrusion alarm signal is generated.
Increasingly in recent times, passive infrared intrusion detectors are being tampered with and rendered inoperative by unauthorized persons. Typically, for example, an infrared detector is sabotaged so that infrared radiation cannot pass through its entrance window, rendering it blind to radiation of interest, preventing the detection of intruders, and allowing them freely to move about the area without being noticed. Typically, sabotage takes place when the intrusion detector is insensitive, i.e. at a time when it is in a stand-by mode when people are allowed to be present in the space. A known method of sabotage involves covering the intrusion detector with an object such as a piece of cardboard or a screen. This type of sabotage can be readily noticed and remedied by security guards, however. A more sophisticated type of sabotage which is not readily noticeable to security guards involves spraying the entrance window with a spray such as an adhesive or hair spray, for example. These sprays are visually transparent, but are opaque to radiation in the infrared range. They are easy to obtain, and can be sprayed onto the entrance window very quickly. This method of sabotage currently is the most prevalent. For autonomous, automatic detection of such sabotage, recent-model detectors are equipped with a means for detecting sabotage, in particular for monitoring the entrance window. Detection may be immediate, at the time of the sabotage in the insensitive state, or only in the sensitive state of the detector.
An intrusion detector of this type is disclosed in European Patent Document EP-0499177, for example. For sabotage detection, this detector has an active radiation source on one side of the entrance window, whose radiation is transmitted through the entrance window and received by a sensor on the other side of the window. The electrical signal output by the sensor is evaluated by a circuit. The radiation is used to measure the optical transmittance of the entrance window and to monitor the space immediately in front of the entrance window for the presence of objects. The characteristics of this radiation are chosen so as not to interfere with the normal operation of the intrusion detector, i.e., the detection of infrared body radiation. If the intrusion detector is sabotaged by covering or spraying, the amount of radiation received by the sensor is increased or decreased. If the sensor signal is outside a predetermined range, a sabotage alarm signal is generated. The radiation source typically is an LED which radiates in the near-infrared region. As a spray used for sabotage is partially transparent in this region, the change in signal strength due to sabotage is small, and the sabotage alarm signal is unreliable. A similar intrusion detector is disclosed in European Patent Document EP-0189536, where a resistor is used as a radiation source which simulates thermal body radiation. Power consumption of the resistor is quite high.